It is known that a digital data signal can be superimposed onto a video signal in such a way so as not to significantly impair conventional reception of the video signal by conventional video signal receivers. The resulting combined signal can be transmitted both to conventional video signal receivers and to specially configured receivers capable of extracting the digital data signal from the video signal. A user with a conventional video signal receiver may, in certain instances, notice some picture degradation in the form of a section of the visible video picture being "snow", but otherwise, the video signal produces a viewable picture. Such a system is described in U.S. Pat. Nos. 4,920,503 and 5,251,301, both to Cook.
There are some drawbacks to combining a digital data signal with a video signal in this manner. For example, in a conventional video transmission system, such as a television broadcast station, there are many recipients of the data. Therefore, it may be impractical to allow the recipients to provide data back to the transmitter indicating, for example, the configuration of the receiver and/or non-receipt of certain portions of the data. For all practical purposes, the communication in such a system is one-way from the transmitter to the plurality of receivers.
In addition, in a conventional communication system, the transmitter can retransmit the communication signal upon receipt of an indication that one or more of the receivers did not properly receive the data that was sent. In the case of a video transmission system, however, this may be impractical, especially where the transmitter is a conventional television station and many of the recipients are receiving and using the conventional video signal portion of the transmitted signal in order to view the picture and audio transmitted therewith in a conventional manner. Such users, who may not be interested in the digital data being superimposed onto the video signal, would probably not appreciate having the same video signal retransmitted numerous times at the request of some of the receivers.